The acid part of a sulfuric acid plant comprises the drying of the sulfur-dioxide (SO2)-containing gases and the air as well as the absorption of the sulfur trioxide (SO3) formed in the contact plant for forming the end products sulfuric acid and oleum. The standard configuration for drier and absorber is the packed tower constructed as counterflow apparatus. This upright cylindrical container is divided into three zones. The lower part is formed by the sump with the acid outlet arranged directly above the bottom or also in the bottom and the gas inlet welded into the sheet metal jacket above the acid level. The middle part contains the packed bed on a grid through which the acid, which is uniformly distributed by a sprinkling system located on the bed, trickles downwards. Above the packed bed the acid inlet to the sprinkling system and the gas outlet are located. Packed towers for the most part consist of a cylindrical sheet metal jacket with a multilayer acid-resistant brick lining and with an arched bottom and a grid of acid-resistant ceramic material for accommodating the packed bed. As grid a domed grid with at least 40% free flow cross-section is incorporated, which is suitable for tower diameters up to 15 m. For the packed bed, saddle packings of ceramic material chiefly are chosen, which have large specific surfaces, whereby an intensive mass transfer between acid and gas is given. (cf. Winnacker/Küchler, Chemische Technik: Prozesse und Produkte, edited by Roland Dittmeyer et al, Vol. 3: Anorganische Grundstoffe, Zwischenprodukte, p. 100 ff, Wiley-VCH Verlag Weinheim, 2005.) A corresponding packed tower also is shown for example in DE 33 20 527 C2.
Because of the deteriorating mounting quality of the brick lining, non-brick-lined stainless steel towers of silicon (Si)-containing stainless steel have increasingly been used for many years. It was found that at several points of the packed tower corrosion problems occur due to waste acid (drier) or oleum formed (absorber). Particularly problematic zones are found in the region of the gas inlet or opposite the gas inlet, where the acid film on the tower wall is only thin. Corrosion also occurs at vertical props for the stainless steel grid or below the packed bed on horizontal surfaces on which the acid film also is only thin. Due to the moisture of the gas in the drying tower waste acid is produced, which is highly corrosive and attacks the steel surfaces. In the absorber towers oleum is produced by the SO3 in gas, which likewise is highly corrosive. In the past, this problem has been countered for example in that around the gas inlet a covering plate of silicon-free austenitic stainless steel is provided, which due to the strong corrosion must however regularly be replaced.